1. Field
The present invention relates to optical communication equipment and, more specifically but not exclusively, to multiple-input multiple-output (MIMO) signal equalization.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
A coherent optical-detection scheme is capable of detecting not only the amplitude of an optical signal, but also the signal's phase. These capabilities make coherent optical detection compatible with the use of spectrally efficient modulation formats, such as quadrature-amplitude modulation (QAM) and phase-shift keying (PSK) in their various forms. Compared to non-coherent optical detectors, coherent optical detectors offer relatively easy wavelength tunability, good rejection of interference from adjacent channels in wavelength-division-multiplexing (WDM) systems, linear transformation of the electromagnetic field into an electrical signal for effective application of modern digital-signal-processing techniques, and an opportunity to use polarization-division multiplexing (PDM).
A conventional coherent optical receiver usually includes a signal equalizer configured to reduce the adverse effects of certain transport-link impairments, such as chromatic dispersion and polarization-mode dispersion. However, the end-to-end transmission of data may cause additional impairments at the transmitter and/or the receiver. For example, the transmitter side can subject the signal to the effects of a skew between the in-phase (I) and quadrature (Q) signal components, radio-frequency (RF) crosstalk between the electrical signals that drive the electro-optical modulator (EOM), and an imperfect common-mode bias in the EOM. In addition, the various digital-to-analog (D/A) converters and driver circuits in the X- and Y-polarization branches of the transmitter can have noticeably different impulse responses. The receiver side can subject the signal to the effects associated with at least some of the aforementioned and/or other additional impairments. Furthermore, in an optical communication system with additional MIMO dimensions, such as a system that employs multimode fibers and space-division multiplexing, a relatively large number of additional circuit elements may introduce other types of signal impairments.